Praise be to Allah.
The best thing to do is to fast on behalf of your mother, because the Prophet (blessings and peace of Allah be upon him) said: “Whoever dies owing any fasts, his next of kin should fast on his behalf.” Saheeh -- agreed upon. The next of kin is the relative. But if you or any other relatives cannot fast, then feed the poor from her estate or from your wealth, one poor person for each day. The amount to be given is half a saa‘ of the local staple food. If you do it all in one go and give it to one poor person, that is also acceptable.
And Allah is the source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and Companions. End quote.
Standing Committee for Academic Research and Issuing Fatwas